The metabolism of inorganic arsenic (iAs) in humans yields methylarsenic (MAs) and dimethylarsenic (DMAs) metabolites that contain arsenic in either +3 or +5 oxidation states. Thus, both trivalent and pentavalent arsenicals are products of the methylation pathway: iAsV -> iAsIII -> MAsV -> MAsIII -> DMAsV -> DMAsIII. The key enzyme responsible for the methylation of iAsIII and MAsIII in this pathway, AsIII methyltransferase, has recently been purified and characterized. Methylated pentavalent arsenicals, MAsV and DMAsV, are less toxic than either iAsV or iAsIII in acute experiments. However, because methylated trivalent arsenicals, MAsIII and DMAsIII, are far more potent than iAs as enzyme inhibitors, cytotoxins, genotoxins and as activators of certain cell signaling pathways, the methylation of iAs can be viewed as a process that potentiates toxicity and carcinogenicity of iAs. A variety of evidence indicates that liver is the main site for enzymatic methylation of iAs in humans. Preliminary studies show that MAsIII and DMAsIII are products of iAs methylation in human hepatocytes, indicating that the hepatic metabolism is the source of MAsIII and DMAsIII that are found in urine of individuals chronically exposed to iAs. The rates of the iAs methylation and yields of MAs and DMAs (including MAsIII and DMAsIII) as estimated from the urinary profiles vary considerably among individuals. The susceptibility to the toxic and carcinogenic effects of iAs also vary widely. Several factors may influence the capacity to methylate iAs, including sex, ethnicity, age, and smoking. However, the mechanism underlying this variation is not well understood. Given the central role of the liver in iAs metabolism, this variation might reflect interindividual differences in the capacity of the liver to methylate iAs and in the production and distribution of the toxic methylated trivalent metabolites. We will examine interindividual variation in the metabolism of iAs in samples of normal human liver (from biopsies, resections and nontransplantable organs) and/or in primary cultures of normal hepatocytes from at least 200 donors during a five-year period. The methylation patterns for iAs, including rates of the production and distribution of methylated metabolites, will be examined for each donor. The maximal capacity of livers to methylate iAs will be determined. The expression levels and genotypes for AsIII methyltransferase will also be characterized. Finally, the susceptibility of cells to arsenicals will be examined using enzymatic, redox and cell viability markers. The toxic effects of iAs will then be evaluated in conjunction with the metabolic, gene expression and genotype patterns. This study will provide unique data on the interindividual differences in the hepatic metabolism and toxicity of arsenic and will link these differences to the expression and genetic polymorphism of the key enzyme in the iAs metabolism.